The revelation that top secret documents relating to al-Qaida and Iraq were left on a train is the latest in a string of recent embarrassments over data security lapses.

In March, it was disclosed that more than 1,000 government computers had been lost or stolen in recent years. The Ministry of Defence was the worst offender, with 503 laptops or PCs missing in the past decade.

On January 19 this year, it emerged that a Royal Navy officer's laptop containing the details of 600,000 people had been stolen. The computer was taken on January 9 in Birmingham and reported the following morning.

It contained personal information from people who had signed up or applied to join the Royal Navy, Royal Marines and Royal Air Force.

The defence secretary, Des Browne, later told the Commons that an investigation into its loss uncovered two similar thefts since 2005.

On December 17, the transport secretary, Ruth Kelly, announced that names, addresses and phone numbers of 3 million candidates for the driving theory test had gone missing.

They were among details on a computer hard drive which went missing in the US in May last year. The hard drive belonged to a contractor working for the Driving Standards Agency.

On December 11, the Department of the Environment revealed that two computer discs containing the names and addresses of 7,685 Northern Ireland motorists had disappeared at a sorting centre in Coventry.

The information was not encrypted. The material had been sent from Northern Ireland Driver and Vehicle Agency in Coleraine, Co Londonderry, and the Driver and Vehicle Licensing Agency in Swansea.

The details included names, addresses, dates of birth, child benefit numbers, national insurance numbers and bank or building society account details.

Despite numerous investigations and repeated pledges to tighten procedures, such data lapses have been common over the years.

In 2004, sensitive papers detailing security arrangements for Heathrow were found near the perimeter and handed to the Sun newspaper.

In 2002, confidential documents detailing protection arrangements for the then home secretary, David Blunkett, were found outside a pub.

In 2000, a MI6 laptop was mislaid after a secret service official went on a drinking session at a London tapas bar.

Less than a month earlier a MI5 laptop containing classified information on Northern Ireland was stolen at Paddington station and in May that year, another laptop was stolen at the same station. It contained details on the biggest ever military procurement project.